Mammalian L1 elements (LINE-1) replicate by copying their RNA transcripts into DNA which is then integrated into the genome. In the last 15 million years this process has generated approximately 550,000 kb of L1 DNA in murine genomes, approximately 20% of the total. In humans L1 retrotransposition causes up to 0.2% of the genetic defects. The approximately 7 kb L1 element has four regions: a 5' untranslated region, (UTR); two open reading frames (ORFs I and II); a 3' UTR. The 5' UTR has a regulatory function, the ORF I protein binds RNA, the ORF II protein is a reverse transcriptase, and the 3' UTR forms complex intrastrand DNA and RNA structures. L1 elements evolve rapidly and a large number of both past and present replicatively successful L1 subfamilies now exist. This complex subfamily structure of L1 DNA permits a comparative approach to the functional analysis of the L1 genome and for determining the natural history of L1 elements. We recently found that ancestral L1 sequences are recycled to create novel modern L1 elements. This could be explained by the reverse transcriptase switching between RNA templates during replication. To test this explanation and explore other aspects of L1 biology we developed an L1 retrotransposition assay in yeast in collaboration with Dr. Leslie Derr (NIAID). Our initial results are most promising, clearly showing that L1 DNA greatly stimulates retrotransposition of a yeast indicator gene. Different deleted versions of the 5' UTR differ greatly in their stimulatory activity when fused to a indicator gene yet promote synthesis of the same amount of RNA. This indicates that a post transcriptional mechanism is involved in L1 regulation. Electroporation of transcripts into cells confirmed this conclusion and showed that L1 5' UTR does not act as an internal ribosome entry site (or IRES). We are now determining the basis of the L1 post transcriptional mechanism. Or work relating the evolution of L1 DNA subfamilies and that of their mammalian hosts has produced several major advances. We were able to date unambiguously the speciation time of modern mammals using the age of L1 subfamilies. This cannot be done with any other molecular character and provides information essential for interpreting both comparative studies as well as very basic biological studies on mammalian evolution. These studies also greatly increased our knowledge on the evolutionary changes in L1 sequence that were compatible with replicative success. This is essential for understanding the epidemiolgy of L1 elements in mammals from which we deduce both the biological properties of L1 and their ultimate affect on their mammalian hosts.